This invention relates to methods of preparing environmentally stable bonded aluminum structure and more particularly relates to methods of preparing bonded aluminum structures in which the aluminum surface is rendered especially well adapted to receive the adhesive resin and is resistant to subsequent delamination and failure of the adhesive bond at the adhesive resinaluminum interface.
It is well known that aluminum or aluminum alloy surfaces exhibit unpredictable and unreliable adherence to bonding media particularly in moist and salt laden atmospheres. It has been proposed to increase adherence of surface coating such as electroplated metal on aluminum base by means of an anodic treatment in an acid bath and then dissolving a portion of the oxide film in an acid or alkaline bath prior to electroplating. See U.S. Pat. No. 1,971,761. It has also been proposed to electroplate directly over an oxide film produced by anodizing aluminum or aluminum alloys in chromic acid of phosphoric acid solution without intermediate treatment of the oxide film such as is taught in U.S. Pat. Nos. 1,947,981 2,036,962 and 2,095,519. In each of the above-noted patents the aluminum surface is being prepared for electroplating.
Similarly, it has been proposed to form anodic coatings having improved adhesive properties on aluminum surfaces by depositing coatings on the aluminum substrate by subjecting the aluminum article to electrolytic treatment in an aqueous solution of various acids such as phosphoric acid, oxalic acid, sulphuric acid, malonic acid and the like at elevated temperatures for a very short treatment period. Similarly, it is known to treat oxides already formed on an aluminum surface by other means with a phosphate bath electrolysis to render the oxide surface hydration resistant. The elevated temperature phosphoric acid anodization process results in the deposition of an oxide surface characterized as "pseudoboehmite", a highly active form of aluminum oxide. The characteristics of this form of aluminum oxide apparently permit failure within the oxide structure when high stressed under humid conditions. The treatment of existing oxide films with phosphoric acid anodization results in a film which apparently lacks coherency and stability at the oxide-metal interface and as a result is inadequate for the formation of an environmentally stable adhesive bond.